1. Field of the Invention
The present invention relates to an image forming device, and more particularly relates to an image forming device which has a laser beam emitting portion for emitting a laser beam according to a driving electric current and forms an electrostatic latent image on an image carrier by deflecting and scanning the laser beam emitted from the laser beam emitting portion.
2. Description of the Related Art
In a conventional image forming device such as a laser beam printer, a copier, or the like, a laser beam is emitted and not emitted by a laser exposure device in accordance with an output image signal, and the uniformly-charged surface of a photosensitive body is exposed by the emitted laser beam so that an electrostatic latent image is formed on the photosensitive body. This electrostatic latent image is developed by toner development and the obtained toner image is transferred onto a recording medium such as a sheet of paper, or the like, and is then fixed to the recording medium. Thus, an image is formed by such a so-called electrophotographic system.
A semiconductor laser (a laser diode, hereinafter called an LD) is generally used as the light source of the laser exposure device for forming such an image on a photosensitive body. In this LD, the laser beam is modulated and outputted in accordance with an image to be formed. A light receiving element is provided at an image-write side on a deflecting-scanning optical path, and horizontal synchronization is established by detecting the laser beam passing through this light receiving element so that writing timing of the image (namely, the timing of the emission of the laser light) is determined.
After the laser beam is deflected by a rotary polygon mirror, the beam diameter and the like are corrected by an optical system. While the horizontal synchronization is established by the light receiving element, the laser beam is focused and is scanned on the photosensitive body so that the latent image of a desired output image is formed on the photosensitive body.
In such an image forming device, in order for the light output of the laser beam to be equal to a required light output value, a monitor voltage corresponding to the light output and a desired voltage reference value (Vref) are successively compared with each other, and a driving electric current of the LD is controlled such that the monitor voltage is equal to the voltage reference value. Such control is called automatic light output adjustment (APC: Auto Power Control) and is executed in a general laser printer or a general copier so that a desired output image is formed.
Characteristics of the LD used as a light source will next be described. The relation of the light output to a supplied driving electric current is generally provided as the LD characteristics as shown by the graph of FIG. 6. In FIG. 6, there is no laser oscillation until a certain threshold electric current (Ith). When an electric current exceeding this threshold electric current (Ith) is supplied, the light output L and the magnitude of the supplied electric current I show a linear characteristic (I-L characteristic).
Deterioration of this I-L characteristic will be explained here. This I-L characteristic mainly deteriorates due to the following three factors.
A first factor is a characteristic shift caused by temperature. As shown by the graph of FIG. 6, in accordance with a rise in temperature of the LD, the threshold electric current (Ith) is increased and the slope of the I-L curve in a laser oscillating area slightly decreases.
A second factor is a characteristic shift caused by an external factor. As an external factor applied from the exterior if the LD, for example, an excessive electric current, exceeding a rated electric current, such as a surge, flows into the LD for some reason. Thus, a light output of power corresponding to the excessive electric current value is momentarily turned on within a chip of the LD. At this time, a chip end face is instantly melted. Depending on the degree of melting, same LDs may be broken whereas same LDs may be relatively slightly damaged and able to normally operate in an usual operation.
However, the light emitting efficiency of an LD whose end face has melted is reduced in comparison with its initial state even if there is only slight deterioration. Namely, a large amount of the driving electric current for obtaining the same light output is required in comparison with the initial state (see FIG. 5) and a heating amount is also increased. At this time, as can be seen from the above characteristic of the driving electric current I and the light output L (see FIG. 6), a higher driving electric current is required as the temperature rises. Thus, the light emitting efficiency is further reduced. The deterioration of the LD is advanced by this repetition so that the LD is broken. With respect to the above I-L characteristic, as shown in FIG. 4, the I-L curve gradually becomes horizontal at the supplied electric current (driving voltage) axis side with each application of static electricity. In addition to this, the linearity of the I-L curve is also lost in a high output area. Namely, a characteristic in which the slope of the I-L curve decreases (bows down) in the high output area is exhibited.
A third factor is deterioration with the passage of time. The driving electric current value for obtaining the same light emitting amount gradually increases due to a cumulative time for supplying the electric current to the LD. Similarly to the aforementioned state of deterioration caused by an external factor, the curve of the I-L characteristics also shows a characteristic of gradually becoming horizontal as shown by the graph of FIG. 5.
Further, in addition to these three factors, there is also dispersion in LD elements themselves.
The following monitoring techniques of LD life span have been proposed conventionally. For example, the following technique is described in Japanese Patent Application Laid-Open (JP-A) No. 61-166085. Namely, an upper limit value of the driving electric current value for obtaining a required light output is set in advance in a printer, a copier, or the like. This driving electric current value is monitored when the driving electric current value is increased to obtain the required light output. When the driving electric current value exceeds the upper limit value, a judgement is made that it is the life span the LD and a warning is given.
Japanese Patent Application Laid-open (JP-A) No. 2-98462 describes the following technique. Namely, a threshold electric current value at a power turning-on time is stored. Further, at an image forming processing time, the driving electric current value at the time when a light output value of 80% of a maximum value of a required light output for image forming processing is obtained is stored. A judgement is made that it is the life span of the LD when the difference between the driving electric current value and the threshold electric current value is larger than a predetermined reference value for judgement.
However, the LD life span is judged on the basis of a preset stored driving electric current value in the technique proposed in Japanese Patent Application Laid-Open (JP-A) No. 61-166085. Therefore, it is difficult to detect an exact deteriorated state due to influences such as a change in temperature within the image forming device during operation of the apparatus, dispersion of each apparatus, and the like. In the technique proposed in Japanese Patent Application Laid-Open (JP-A) No. 2-98462, the difference between the two electric current values at the power turning-on time and the image forming processing time is determined, but there is a high possibility of different temperatures within the image forming device at the power turning-on time and the image forming processing time. Accordingly, the influence of the change in temperature within the image forming device is included in the difference between the two electric current values. Therefore, it is difficult to detect the exact deteriorated state.
Further, in the prior art examples, only the warning of the LD life span is given in advance, but the life span of the image forming device cannot substantially be extended when the LD is deteriorated by a certain cause. In actuality, even if the warning of the LD life span is given in advance, if the light output at the initial light amount setting time of the LD is not increased up to the prescribed light amount required to write an image, the image forming function is stopped at that time, and the image forming device cannot operate normally until a built-in optical scanner or the like is replaced.
When the initial light amount of the LD is adjusted, the driving electric current is adjusted such that the light output becomes equal to a desirable value, while the driving electric current is increased by a predetermined unit light amount adjusting width. As the deterioration of the LD is advances, the slope of the above-discussed I-L curve decreases. Accordingly, the number of adjustments of the initial light amount of the LD increases.